More broadly: consider genetic drift and the probability of reaching fixation. For neutral mutations, their probability of fixation is the rate at which they are introduced, and they will reach fixation at 4*population-size generations. For primate species, the population size is always pretty small, low hundreds of thousands or millions; generation turnover tends to be something like 10 years, and early primates can date back as much as 60 million years, so it can encompass a lot of drift. If we imagine that kin altruism is neutral until you have at least a few relatives and the relevant mutation keeps happening once in every few hundred thousand individuals, it’s not at all unlikely that it will appear repeatedly and then drift up to the threshold where fitness gains start appearing, and then of course, now that it’s no longer neutral, it’ll be quickly selected for at the rate of its gain.
More broadly: consider genetic drift and the probability of reaching fixation. For neutral mutations, their probability of fixation is the rate at which they are introduced, and they will reach fixation at 4*population-size generations. For primate species, the population size is always pretty small, low hundreds of thousands or millions; generation turnover tends to be something like 10 years, and early primates can date back as much as 60 million years, so it can encompass a lot of drift. If we imagine that kin altruism is neutral until you have at least a few relatives and the relevant mutation keeps happening once in every few hundred thousand individuals, it’s not at all unlikely that it will appear repeatedly and then drift up to the threshold where fitness gains start appearing, and then of course, now that it’s no longer neutral, it’ll be quickly selected for at the rate of its gain.